Unified Role of the 145th Residue on the Fluorescence Lifetime of Fluorescent Proteins from the Jellyfish <i>Aequorea victoria</i>

Finding a unified fluorescence mechanism is essential to develop and utilize fluorescent proteins appropriately. Here, we report the unified role of the 145th residue on the fluorescence efficiency of fluorescent proteins developed from the jellyfish <i>Aequorea victoria</i> by demonstrating the difference and similarity between two representative fluorescent proteins, enhanced green fluorescent protein (eGFP), and enhanced yellow fluorescent protein (eYFP). We determined the fluorescence lifetimes of the 19 different Y145 mutants of eGFP and eYFP by picosecond time-resolved fluorescence spectroscopy. We found that the effect of the 145th mutation on the fluorescence lifetime is significant for eYFP but moderate for eGFP. We compared known crystal structures to clarify the observed difference between eGFP and eYFP. As a result, we conclude that the efficiency of the steric restriction of the chromophore motion by the 145th side chain is essentially the same for both eGFP and eYFP. Meanwhile, the restriction of the chromophore motion by hydrogen bonds is more pronounced for eGFP than for YFP. Balance of the steric effect and hydrogen bonding controls the lifetime of the Y145 mutants for eGFP and eYFP. Furthermore, the steric restriction is induced by the electrostatic effect; the different 145th residue induces a different electrostatic environment around the chromophore. The finding in this study reasonably explains the reported lifetimes of other fluorescent proteins and allows the prediction of the lifetime of unknown fluorescent proteins from jellyfish.